Alkanolamine sweetening units are used for the removal of H.sub.2 S and CO.sub.2 from natural gases, enhanced oil recovery gases, refinery hydrodesulfurizer recycle gases, FCCU and Coker gas plant tail gases, LPG streams, and Claus sulfur recovery tail gases. The alkanolamines commonly used are ethanolamine, diethanolamine, methyl diethanolamine, diisopropanol amine, and triethanol amine. These compounds are weak bases in water solution. When solutions of alkanolamines are contacted in packed, sieve plate, bubble cap, or valve tray columns with streams containing H.sub.2 S and CO.sub.2, the H.sub.2 S and CO.sub.2 dissolve into the alkanolamine solution. The following chemical reactions then take place: ##EQU1##
The solution of water, unreacted alkanolamine, and alkanolamine salts are subjected to steam stripping to decompose the alkanolamine salts and remove H S and CO.sub.2 from the alkanolamine. The H.sub.2 S and CO.sub.2 removed from the alkanolamine can then be processed by Claus sulfur recovery, incineration, fertilizer manufacture, or other means.
H.sub.2 S and CO.sub.2 are not the only gases in the above referred to streams which form weak acids when dissolved in water. Other such acid gases, as they are commonly called, that may appear in gas streams treated with alkanolamine include SO.sub.2, COS, or HCN. These gases also undergo the same reactions as H.sub.2 S and CO.sub.2 to form alkanolamine salts. These salts, though, cannot be removed by steam stripping as are H.sub.2 S and CO.sub.2 salts. Thus, they remain and accumulate in the system.
Another problem is presented if oxygen gets into the alkanolamine system. Oxidation of acid gas conjugate base anions leads to the formation of other alkanolamine salts, most commonly salts of thiosulfate (S.sub.2 O.sub.3.sup.-2), sulfate (SO.sub.4.sup.-2), thiocyanate (SCN.). Other inorganic acid anions, such as, chloride (Cl.sup.-) may also be present. These salts also cannot be regenerated by steam stripping.
Alkanolamine salts which cannot be heat regenerated, called heat stable salts, reduce the effectiveness of alkanolamine treating. The alkanolamine is protonated and cannot react with either H.sub.2 S or CO.sub.2 which dissolve into the solution. Also, accumulated alkanolamine salts are known to cause corrosion in carbon steel equipment which is normally used in amine systems. The salts are also known to cause foaming problems which further decreases treating capacity.
The normal procedure used to deprotonate the alkanolamine, so it can react with H.sub.2 S and CO.sub.2, is to add an alkali metal hydroxide, such as NaOH, to the amine solution. The deprotonated alkanolamine then can be returned to H.sub.2 S and CO.sub.2 removal service. However, the sodium salts of the anions of the heat stable salts are also heat stable, are difficult to remove and thus accumulate in the alkanolamine solution, with attendant corrosion and foaming problems.
In one process, the alkanolamine solution containing heat stable alkali metal salts is contacted with a basic anion exchange resin to remove the heat stable anions from the solution and thereafter the solution is contacted with an acidic cation exchange resin whereby alkali metal ions are removed from the solution. Anions of any heat stable alkanolamine salts are also removed by the basic anion exchange resin. Removing the heat stable salts in this manner reduces foaming losses. corrosion and maximizes the alkanolamine concentration.
The basic anion exchange resin used in the described process is regenerated by flushing with water to remove free alkanolamines, followed by elution with dilute sodium hydroxide to displace heat stable salt anions with hydroxide ions and a second water wash to remove residual sodium hydroxide and sodium slats. The acidic cation exchange resin is regenerated by flushing with water to remove free alkanolamine, followed by elution with dilute hydrogen chloride to displace sodium cations with hydrogen ions. A second water wash is then used to remove residual hydrogen chloride and sodium chlorides.
In the described process alkanolamine in the alknaolamine solution is protonated by hydrogen at the ionic sites on the acidic cation resin and becomes attached to these sites as alkanolamine cations. When the cation resin is regenerated with the dilute HCl solution, both alkali metal cation and such alkanolamine are displaced from the resin, with hydrogen ions taking their place. The alkanolamine in the regenerant stream cannot be returned to the alkanolamine circulating system for reuse because the alkali metal and chloride ions in the regenerant would recontaminate the system. The resultant loss of alkanolamine is unacceptable both economically and environmentally.